Bass Boost on MCM 250 W Plate Amp (1 Viewer)

[Rick Thomas]

Ron how does your sub sound with the bass boost enabled?

 

[Ron-P]

See this thread I've got going. So far I am so-so impressed, but I have issues. Just need to find the cause and fix them.
http://www.hometheaterforum.com/htfo...threadid=64421
On a side note. I just spoke with Brian at Adire and he said they do not remove the bass boost resistors, but rather just piggy back them with another to change the boost. Anyone know about doing it this way?
Peace Out~

 

[Jim A. Banville]

I think that if you have a 500K resistor, but you need the resistance to be 250K, you could "piggy back" (a.k.a. run in parallel) a 125K resistor over the 500K one and you'd end up with 250K of resistance.

 

[TommP]

Ron,

Piggy backing the resistors would work fine, but it was very simple to just desolder the existing resistors and solder in the new ones.

 

[Patrick Sun]

Actually if you parallelled a 500K ohms resistor with a 125K ohms resistor, you get the equivalent of 100K ohms resistor.

To get 250K ohms, you need to parallel/piggyback another 500K ohms resistor to the initial 500K ohms resistor.

Rtotal = (Ra * Rb)/(Ra + Rb) (for 2 resistors in parallel)

For n number of resistors:

1/Rtotal = 1/Ra + 1/Rb + ... + 1/Rn

I think I'll be doing the (zero boost, F3=12Hz) mod, which just means changing the R26 from 18K ohms to 100K ohms.

But I do like the (almost zero boost, F3=16Hz) mod which means changing R26 to 50K ohms, and R27 to 150K ohms.

Resistors can be bought at Radio Shack, 1/4W variety. 1/2W resistors are also okay, just more expensive.

 

[Brian Bunge]

Jim,

I think that if you have a 500K resistor, but you need the resistance to be 250K, you could "piggy back" (a.k.a. run in parallel) a 125K resistor over the 500K one and you'd end up with 250K of resistance.

No, that will result in a resistance of 100K ohms. The formula for two resistors in parallel is:

(R1*R2)/(R1+R2)

So in your example, if you had a 500K resistor and needed a value of 250K, you'd simply add a second 500K resistor in parallel with the existing one. Also, for multiple resistors of the same value paralleled simply divide the resistance value by the number of resistors.

EXAMPLES:

Two 500K resistors so 500K/2=250K

Four 8 ohm voice coils is 8/4=2

and so on...

Brian

 

[Jedd]

Opened it up, here what I have:
r26:
brown;grey;orange;bronze
r27:
bronze;grey;brown

 

[Patrick Sun]

http://www.rfcafe.com/references/ele...r_markings.htm
So...R26=18K ohms, R27=180K ohms (you have the stripes backwards for R27).

 

[Brian Bunge]

Dammit Pat! You beat me to it! But at least we both were right.
It's always nice when a GA Tech grad and a DeVry grad can agree!
Brian

 

[Ron-P]

Brian at Adire sent me this mod that they do in their factory.
Add resistor to R27 - 680k ohm
Add capacitor to C14 - .47 uF
Any comments if this is a good fix?
Peace Out~

 

[Patrick Sun]

Ron, if you're happy, then do nothing, but if you want to try the mod, give it a shot. These Adire folks know what they are doing, so it can't hurt (you can always take the mod parts off pretty easily). The mod is decreasing the R27 resistor, and increasing the capacitance of C14.

 

[Ron-P]

I am very happy right now, but I think I will try the mod anyway. Brian is going to send me the parts. Like you said, I can easily remove them if I do not like the results.
Do you think my 40watt soldering iron will be too hot?
Peace out~

 

[Patrick Sun]

Yes, you need to use a 15W soldering iron, 25W at the most (be quick and nimble).

 

[Pete Mazz]

Ron,

I would take a FR before you do the mod. You never know till you see the numbers....you may want the boost.

Pete

 

[Jedd]

Time to solder my amp too

so, does R26 = 56kOm, R27 = 120kOm mean the same with R26=100kOm, R27 = 180kOm with bass boost = 0?

Should I left some bass boost for sealed 150L Dayton 15" enclosure (generally speaking) or I better remove it completely? What system is more sensetive to the bass boost ported or sealed?

 

[Jim A. Banville]

Ron-P, did you modify your amp with the parts from Adire yet?

thanks

Jim

 

[Ryan Schnacke]

so, does R26 = 56kOm, R27 = 120kOm mean the same with R26=100kOm, R27 = 180kOm with bass boost = 0?

Not quite the same thing. Both of these configurations are approximately a Butterworth hi-pass filter which means that they both have no boost. The difference is where the hi-pass cutoff is located.

For R26 = 56kOhm, R27 = 120kOhm, Fc = 19.4Hz

For R26 = 100kOhm, R27 = 180kOhm, Fc = 11.9Hz

So you just have to decide where you want to start rolling off the amplification. Those numbers are pretty far apart so lets see if I can't come up with something in between:

For R26 = 75kOhm, R27 = 160kOhm, Fc = 14.5Hz

For R26 = 75kOhm, R27 = 150kOhm, Fc = 15Hz

For R26 = 62kOhm, R27 = 130kOhm, Fc = 17.7Hz

For R26 = 62kOhm, R27 = 120kOhm, Fc = 18.5Hz

These are all very close to Butterworth response so they have no boost.

 

[Ryan Schnacke]

Here's something interesting - when I run the numbers, most of the PE recommended resistor combinations give me approximately what they claim. Usually +/- 0.5dB and within the specified frequency range (though sometimes near the edge of that range).

Now I understand that PE could only specify resistor values that are readily available (as I did in my post above), so their values won't always give you EXACTLY the claimed boost and EXACTLY the center of the frequency range. But when I run the numbers for the default values on the 120watt amp (18k and 180k) I get 4.4dB of boost at 31Hz. Now why would they care to claim 6dB at 30Hz? They certainly could achieve this if they wanted to by using 15k and 220k. But that's not what they're using. Maybe a mistake?

 

[Jedd]

Ryan, thanks for explanations!
So far I decided to make 56kOm&220kOm mod to give 1db boost in 20-24Hz range, so it wil help room response to keep curve flat. But even 1db boost drives Tempest very close to Xmax with 50W at 22Hz:
http://www.hometheaterforum.com/htfo...threadid=67037
6dB is insane, after all where did we get this number from? It sounds like a good way to make a hole in the driver.

 

[Patrick Sun]

If anyone cares, I went with R26=39K and R27=150K for my final amp setting of the bass boost (1dB around 31-35Hz)/rumble filter (Fc=21Hz).

It helped keeping the driver in control on many bass-heavy DVDs.